Many electronic systems require memories in which the data are fixedly written. Memories of this sort are designated, among other designations, as fixed-value memories, read memories or read-only memory.
For very large quantities of data, plastic disks coated with aluminum are often used as read memories. In the coating, these plastic disks have two types of punctiform recesses, allocated the logical values zero and one, The information is digitally stored in the arrangement of these recesses. Disks of this sort are called compact disks and are distributed for the digital storage of music.
To read the data stored on a compact disk, a reading device is used in which the disk rotates mechanically. The punctiform recesses are scanned via a laser diode and a photo cell. Typical scan rates are thereby 2.times.40 kHz. Five gigabits of information can be stored on a compact disc.
The reading device has moving parts that are subject to mechanical wear, require a relatively large volume and permit only a slow access to data. In addition, the reading device is sensitive to vibration, and its usability in mobile systems is thus limited.
For storing smaller amounts of data, fixed-value memories based on semiconductors are known. These are often realized as planar integrated silicon circuits in which MOS transistors are used. The MOS transistors are respectively selected via the gate electrode connected with the wordline. The input of the MOS transistor is connected with a reference line, the output with a bitline. During the read process, it is evaluated whether or not a current is flowing through the transistor. The stored information is allocated correspondingly. Technologically, the storing of the information is mostly effected in that the MOS transistors have different operating voltages due to different implantation in the channel region.
These semiconductor-based memories allow freely selectable access to the stored information. The electrical power required for the reading of the information is significantly lower than in a reading device with a mechanical drive. Since no mechanical drive is required to read the information, the mechanical wear is omitted, as is sensitivity to vibration. Semiconductor-based fixed-value memories are thus also usable for mobile systems.
The specified silicon memories have a planar construction. A minimal surface requirement is thus necessary per memory cell, about 6 to 8 F.sup.2, where F is the smallest structural size that can be produced in the respective technology. In a 1 .mu.m technology, planar silicon memories are thereby limited to storage densities of about 0.14 bit per .mu.m.sup.2.
In planar silicon memories, it is known to increase the storage density by arranging the MOS transistors in rows. In each row, the MOS transistors are connected in series. The MOS transistors are read out by means of row-by-row driving in the sense of an NAND architecture. For this purpose, only two terminals are required per row, between which the MOS transistors arranged in the row are connected in series. Source/drain regions, connected with each other, of adjacent MOS transistors can then be realized as a continuous doped region. In this way, the surface requirement per memory cell can theoretically be reduced to 4 F.sup.2 (F: the smallest structural size manufacturable in the respective technology). A memory cell arrangement of this sort is known, e.g. from H. Kawagoe and N. Tsuji in IEEE J. Solid-State Circuits, vol. SC-11, p. 360, 1976.
The programming of the specified memory cell arrangements ensues during manufacture. However, many applications require memory into which the data can be written by electrical programming. In electrically programmable memory cell arrangements, the storing of the information is mostly effected in that a floating gate is provided between the gate and the channel region of the MOS transistors, which gate can be charged with an electrical charge, or, as gate dielectric, a double layer of SiO.sub.2 and Si.sub.3 N.sub.4 is provided, on whose boundary surface electrical charge bearers can be held at traps. The operating voltage of the MOS transistor is dependent on the charge located at the floating gate or, respectively, the traps. This characteristic is used for electrical programming (see e.g. S.M. Sze, Semiconductor Devices, John Wiley, p. 486-490).